Many types of electrolytic plating solutions have been developed to deposit chromium electrochemically on a metal substrate. One of the most widely used solutions contains predominantly hexavalent chromium ions [Cr(VI)] in the form of dissolved chromium trioxide (CrO.sub.3). Chromium trioxide is mixed with water and a sulfate catalyst to produce a plating bath that provides a lustrous protective or decorative chromium plate.
It has long been known that a predominantly hexavalent chromium ion solution produces a brighter, more lustrous, thick-plated product than a trivalent solution. Moreover, significant amounts of trivalent chromium have been considered an undesirable contaminant in chromium electroplating solutions.
More recently, U.S. Pat. Nos. 4,447,229 and 4,615,773 disclosed electrolytic plating bath solutions that contained both trivalent and hexavalent chromium. The current efficiency of these electroplating processes was improved by adding small amounts of methanol to a bath containing dissolved CrO.sub.3 electrolyte. This bath promoted rapid electrodeposition of a chromium plate with greater uniformity of the plated product. Particularly good current efficiency was observed when the bath contained dissolved metallic ions, such as iron. Current efficiency also was enhanced by maintaining the pH at the cathode at about 2.0 with a metal ion buffer.
Although chromium plating processes have long been known, the versatility of industrial processes using such plating generally has been limited because chromium softens when heated. Such heat softening is a particular problem in production processes that plate chromium on a heat-hardenable substrate, such as an alloy steel. In the production of cutter elements, for example, it is necessary to heat-harden an alloy steel substrate before electrochemically plating the substrate with chromium. This avoids softening the chromium during a heat treatment step. Moreover, the surface of the steel substrate oxidizes when heated and must be thoroughly cleaned with a caustic material or other cleaning agents prior to plating. If such a cleaning step is not performed prior to plating, the chromium metal does not adhere well to the underlying steel substrate. Hence, the necessity of heating the substrate prior to plating introduces an additional costly step into the manufacturing process.
Another drawback to conventional electrodeposited chromium plate is that hydrogen is evolved at the cathode and incorporated into the chromium metal. Hydrogen can then diffuse from the plated metal into an alloy steel substrate, thereby embrittling the metal alloy. The plated chromium can be heated to 500.degree.-650.degree. C. to evolve hydrogen, thereby avoid such embrittlement, but such heating unacceptably softens the chromium plate. Lower heat treatment temperatures can avoid chromium softening, but require prolonged periods of heating. Hence, prevention of hydrogen embrittlement of the substrate cannot be avoided by heat treatment without concomitantly sacrificing hardness of the chromium plate or prolonging the manufacturing process.
Another drawback associated with trivalent chromium plating processes is that the thickness of the deposited chromium layer has been limited from about 2 .mu.m to about 5 .mu.m. For instance, previous processes employing trivalent chromium have been found to produce chromium layers having a thickness of approximately 3 .mu.m. Where a chromium layer greater than about 3 .mu.m is required, conventional trivalent plating processes have not been able to produce the desired thickness.
Yet another problem encountered in chromium electroplating is that conventional electrolytic baths contain high concentrations of hexavalent chromium ions. Hexavalent chromium ions are extremely toxic. The disposal of hexavalent chromium is subject to strict and costly environmental regulations that greatly increase the expense of electroplating processes.
A final problem associated with previous plating solutions is the inability to plate a substrate with varying percentages of iron and chromium. Hence, it would be helpful to have a plating bath that eliminates the use of hexavalent chromium, produces a heat-treatable substrate coating, and can deposit chromium and iron metal layers having thicknesses of greater than about 50 .mu.m. Such a bath has not been described prior to the present invention.